Crescent gradient coils

ABSTRACT

A high-conductivity ceramic coil form with an internal water jacket is used to simplify water cooling for 3-axis MRI gradient coil configurations on a single cylindrical coilform. Crescent-shaped, axially aligned coils are symmetrically employed on either side of the axial symmetry plane to increase transversely the region of field linearity. These crescent coils may be used in conjunction with Golay-type coils for improved switching efficiency. Lead-filled copper tubing may be used to reduce acoustic noise from pulsed coils in high external magnetic fields.

This application is a divisional of application Ser. No. 08/030,853,filed on Mar. 12, 1993, now U.S. Pat. No. 5,554,929 incorporated hereinby reference.

FIELD OF THE INVENTION

The field of this invention is electromagnetic coils for the purpose ofefficiently generating gradients, especially in magnetic resonanceimaging (MRI) and other gradient techniques employing a superconductingmagnet.

BACKGROUND OF THE INVENTION

Most modern MRI systems use a superconducting solenoid to establish auniform B₀ (or B_(Z)) over the imaging volume. This results in themagnetic field being collinear with the path available for sampleaccess. Coils are then required to produce monotonic, (preferablylinear) gradients in B_(Z) with respect to x, y, and z over the sampleregion during precisely determined pulse sequences. The transversegradients (δB_(Z) /δx, δB_(Z) /δy) in the prior art have generally beenestablished by symmetrically located sets of saddle coils, similar tothose first described by Golay in U.S. Pat. No. 3,569,823 or by relatedplanar coils as disclosed by Roemer, U.S. Pat. No. 4,926,125 and Morichet al, U.S. Pat. No. 5,036,282. Maxwell pairs or related geometries areuniversally used to generate the axial gradient. A co-pendingapplication, Ser. No. 07/912,149, discloses the use of coil geometriesmore complex than surface currents to achieve order-of-magnitudeimprovements in several critical parameters for transverse gradientcoils: acoustic noise and DC gradient efficiency.

The instant application discloses (a) the combined use of Golay-type andcrescent-coil geometries for greatly improved switching efficiency and(b) the convenience of internal water cooling with transverse gradients.The closest prior art to the instant invention, in terms of magneticfield configuration, appear to be the trapezium loops for use with anelectromagnet, as disclosed in the article "Magnetic Field GradientCoils for NMR Imaging" by Bangert and Mansfield in Journal Physics, E,15, 235 (1982), some screening concepts disclosed by Mansfield in U.S.Pat. No. 4,978,920, and the above referenced co-pending patentapplication.

The gradient pulses induce eddy currents and vibrations in nearbyconducting structures (especially in flimsy shields, in the cryostat,and in lightweight rf coils) which perturb the field homogeneityfollowing the pulses with time and spatial dependencies that are noteasily characterized. Actively shielded coils for MRI were firstpublicly disclosed by Mansfield in February 1986 at approximately thesame time that Roemer filed the patent application which resulted inU.S. Pat. No. 4,737,716. Prior independent work was underway at DotyScientific, which shipped the first such commercially available NMRgradient coils in January 1987. Actively shielded dipolar coils forenergy storage were previously disclosed by Westendorp, U.S. Pat. No.3,671,902. Actively shielded, constant-gradient, quadrupolar magneticfield coils based on cylindrical current sheets for atomic beamconfinement and focusing were previously disclosed by Beth, U.S. Pat.No. 3,466,499.

FIG. 1 approximately depicts the fingerprint coils of Schenck, Hussain,and Edelstein, U.S. Pat. No. 4,646,024, as used to generate δB_(Z) /δyin an imaging region in the sample. Such a pattern achieves both higherlinearity and higher switching efficiency than first-order Golay coils.A similar set of concentric coils rotated 90°, is used to generateδB_(Z) /δx.

The major gradient design problems center on the following: (1) limitedavailable space because of economic considerations, (2) motion-inducedartifacts arising from the finite stiffness and mass of the coil supportstructure, (3) practicable coil winding (or etching) techniques, (4)acoustic noise abatement, (5) heat dissipation, and (6) minimization oftransverse field components.

The conflicting technical requirements may be partially addressed bymeans of local planar gradient coils with highly nonlinear response, asdisclosed by Roemer, U.S. Pat. No. 4,926,125. By adding distortioncorrection algorithms to the image processing, it is possible to usegradients with ±40% to ±60% non-linearity on one axis in applicationswhere high spatial resolution is required only over a small portion ofthe image.

The following parameters generally need to be specified for gradientcoil systems: gradient coefficient α (T/Am) (sometimes called gradientefficiency in the prior art), imaging sphere diameter d_(i) (m) for aspecified linearity deviation, inductance L (H), resistance R_(E) (Ω)maximum continuous power dissipation P (W), maximum pulse current I_(P)(A) in a specified B₀, recovery time T_(D) (s) for a specified pulse,acoustic noise for a specified pulse sequence in a specified field, andratio of transverse field energy in the sample region to axial fieldenergy in the imaging region.

For the fastest imaging technique, Echo Planar Imaging (EPI) and relatedtechniques, the most important parameters are recovery time, gradientswitching efficiency, transverse fields, and acoustic noise. AlthoughEPI was first described more than 15 years ago, it has seldom been usedbecause prior art gradient coils (a) may require megawatts of gradientdriver power on the frequency-encoding axis, (b) generate sound pressurelevels that are painful and damaging to the patient's hearing, (c)produce motion-related artifacts that cannot be fully removed even withthe most sophisticated image postprocessing, and (d) require high poweraudio amplifiers costing up to several million dollars. A recentexperimental demonstration at 0.5 T required nearly half a megawatt (at10% duty cycle) at 1 kHz, and others have proposed the use of 2 MW at 5kHz, 1.5 T, and 50% duty cycle for slice-interleaved EPI techniques. Theabove problems may be partially addressed using a tuned transversegradient with sinusoidal (monochromatic) current; but the conventionalgradient coil has very low electrical Q; and there are penalties in SNRand heat dissipation. Also, computational analysis becomes more complex,but the software is available.

While the Maxwell z-gradient is considerably more efficient than theGolay transverse gradient, the frequency-encoding gradient must be inthe plane of the image, which often must be transverse for medicalreasons. Therefore, improvements are needed in transverse gradients.

The image artifact problem can begin to be appreciated by noting thatwhile the frequency-encoding gradient may be driven with a 500 kVAtrapezoidal waveform, the phase-encoding gradient is being driven withshort "blips" of several kilowatts at very low duty cycle, and theslice-selection axis is nulled. It is quite easy for nonlinear,vibration-dependent couplings between the frequency-encoding axis andthe other axes to destroy the required degree of orthogonality betweenthe axes and produce phase-related artifacts.

It should be pointed out that there is ambiguity in the definition andusage of the term "linearity" in the MRI gradient literature.Henceforth, we use this term to indicate the rms deviation of localfield slope compared to the mean field slope over a specified volume.This definition is generally more demanding and a better indicator ofimage quality than the more common definition where linearity is definedas the maximum gradient field deviation relative to a linear field atany point in the sample, as the latter definition averages localfluctuations along the gradient axis. Other definitions can be lessdemanding and less useful.

The availability of better image processing and distortion correctiontechniques suggests that the rms gradient deviation a be increased to14%, compared to the more typical 10% value for many prior artgradients, to increase imaging volume. It is still important that thefield be monotonic, but the method of Schenck et al in U.S. Pat. No.4,646,024 results in relatively poor switching efficiency, intolerableacoustic noise, and unmanageable motion-related artifacts.

The enormous bandwidth (several MHz) of high-resolution EPI (and othermore advanced techniques) can reduce the imaging time by two or threeorders of magnitude without placing unrealistic demands on moderncomputers since computational power per cost has increased at the rateof 40% per year for the past seven years and that rate is expected tocontinue for several more years. Designing for strong gradients withlarger gradient non-linearity with very fast switching places increased(though inconsequential) computational demands on the image processing.While there may be some increased variation in SNR over the final image,this is more than offset by the increased data rate.

In practice, using conventional shielded gradient coils, the inductiveenergy (I² L/2) is larger than suggested by simple energy estimates by asubstantial factor. In a co-pending patent application, methods aredisclosed that allow increases in α² /L by factors of 2 to 10 comparedto prior art Golay coils. However, for large systems the most importantefficiency figure-of-merit often is η_(s) =α² d_(i) ⁴ h_(i) /μ₀ L, whereh_(i) is the imaging region field-of-view in the axial direction. Theinstant invention allows increases in η_(S) by factors of 2 to 20compared to prior art.

Some prior gradient coil designs have also suffered under the falsenotion that there is an inherent advantage with very low inductancecoils. Higher inductance (more turns) requires higher voltage, but nothigher power (VA) for the same switching time. In fact, reducinginductance below 100 μH is detrimental as lead inductance andtransmission line problems then becomes significant. Coil orthogonality(for isolation) and net force cancellation both dictate that integralnumber of turns be used in all coil sets and coil subsets. Hence, theaccuracy of the shielding is limited from this quantization. The moreturns, the more precisely the gradients can be shielded. Optimum numberof turns is thus determined largely by the VA characteristics andeconomics of available power devices, magnetic shielding accuracyrequirements, and standard wire insulation practice, making 250 V to 800V (peak differential voltages for a balanced line) at 20 A to 300 A bestfor large systems. Optimum inductance is typically 0.2 to 1 mH.

SUMMARY OF THE INVENTION

A high-conductivity ceramic coil form with an internal water jacket isused to simplify water cooling for 3-axis MRI gradient coilconfigurations on a single cylindrical form. Crescent-shaped, axiallyaligned coils are symmetrically employed on either side of the axialsymmetry plane to increase transversely the region of field linearity.These crescent coils may be used in conjunction with Golay-type coilsfor improved switching efficiency. Lead-filled copper tubing may be usedto reduce acoustic noise from pulsed coils in high external magneticfields.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the prior-art, shielded, fingerprint, transversegradient coil in the spherical coordinate system;

FIG. 2 discloses a tapered crescent coil with two parallel windings oflead-filled copper tubing;

FIGS. 3a and 3b illustrate a method of winding a coil on a coilform thathas a concave surface;

FIGS. 4a and 4b disclose a transverse gradient coil system using a Golaycoil and three axially oriented crescent coils on each side;

FIG. 5 depicts the use of an internal water cooling jacket;

FIGS. 6a and 6b illustrate a preferred 3-axes gradient coil assemblywith an internal water cooling jacket;

FIG. 7 is a schematic representation of a method of connecting parallelwindings to achieve orthogonality;

FIG. 8 is a schematic representation of a method of achievingorthogonality using gradient coils with shared windings;

FIG. 9 is a cross section of an X-Y gradient coil system using a Golaycoil and twelve axially oriented crescent coils;

FIG. 10 is a schematic representation of a method of orthogonallypowering 12 crescent coils with shared windings; and

FIG. 11 is an embodiment of the invention with a stepped shape.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Qualitative comparisons of various gradient coil geometries of differentsizes are often misleading because of the complex expressions for power,inductance, resistance, and gradient coefficient and the varying degreesof sensitivity to coil motion. For example, power is often proportionalto the radius to the fifth power for constant winding thickness andvoxel size, but power is often linear with radius for constant number ofvoxels and constant relative winding thickness, depending on therelative significance of γGd_(i) T₂ and SNR (signal to noise ratio) inthe determination of spatial resolution. Here, γ is the magnetogyricratio, G is the magnetic field gradient (T/m), d_(i) is the imagingdiameter, and T₂ is the spin-spin relaxation time (s). Thus, it isuseful to develop dimensionless figures of merit for comparison ofvarious gradient coil systems of various sizes.

We define a switching figure-of-merit, or switching efficiency, η_(S) :##EQU1## where α is the gradient coefficient (T/Am), d_(i) is theimaging diameter for 14% rms gradient deviation σ, h_(i) is the axialimaging length for the same deviation, μ₀ is the permeability of freespace, and L is the inductance. The above definition differs from apreviously derived definition by a constant, making it more convenientlyexpressed as a percent. For a typical ellipsoidal imaging region, it hasnumeric value between 1% and 20% for shielded Golay coils and 15% to 90%for shielded Maxwell pairs and quadrupolar coils for use in magnets withtransverse B₀.

Prior-art fingerprint coils as shown in FIG. 1 have typical η_(S) of 10%to 30%, depending primarily on the shielding ratio to gradient radii.The crescent-Golay geometry disclosed in the current applicationachieves η_(S) of 20% to 90%.

It should be noted that the gradient field produced by Golay orfingerprint coils is predominately in the radial direction near window101, positioned close to polar angle θ of 45°. This transverse componentis of no imaging value, but it is responsible for the majority of thecurrents induced in the sample, which are to be minimized for patientsafety reasons. Largely for this reason, and to enhance rf efficiency,it is not practical for d_(i) to exceed 1.7r_(g), where r_(g) is thegradient coil radius, as the induced currents increase rapidly forlarger values of d_(i).

Next we define a low frequency (LF) electrical efficiency η_(L) :##EQU2## where the constant has the units m/s as indicated. Thisdefinition differs from an earlier definition by a constant factor,primarily for convenience. This LF efficiency evaluates to 2.5% for atypical prior-art transverse coil designed for d_(i) =84 mm (the copperthickness was about half the skin depth) but values below 1% are typicalfor large planar transverse gradient coils. Typical values forMaxwell-pairs are near 10%, and there is usually little justificationfor higher LF efficiency, although values above 40% can be achieved fortransverse gradients with octopolar geometries of the co-pendingapplication and crescent-Golay systems of the current application.

The figure of merit governing coil power dissipation during EPI isQη_(S), but the electrical Q at the switching frequency (e.g., 1600 Hz)cannot easily be determined, except by experimental measurement. Theelectrical Q is generally proportional to coil volume and the squareroot of frequency. For shielded whole-body coils at 1600 Hz it istypically 3 to 30.

Optimum conductor thickness in the fingerprint coil in regions where thegradient field is predominately axial is approximately one skin depth(typically 3 mm for copper) at the EPI switching frequency. However,optimum thickness in the vicinity of the window, where large radialcomponents are present, is greater.

Coil motion is one of the most troublesome design limitations in manyprior art gradient coils. Golay-type transverse gradient coils in auniform external magnetic field develop opposite torques which cause thecylindrical coilform to bow in the plane of the z-axis and the desiredgradient. The governing equations change radically depending on whethermost of the energy in the gradient pulse spectrum is below or above thefundamental mechanical mode to which it is strongly coupled.

We define a high-frequency electro-mechanical efficiency η_(mh), (whichwe want to minimize) as the ratio of mechanical energy in the coils tomagnetic energy in the sample and show that for conventional Golay coilsit is approximately as follows:

where t_(g) is the gradient pulse length (s), s is the ##EQU3##difference between the shield coil radius and the gradient coil radius(m), and m_(c) is the Golay coil mass per quadrant (kg). Clearly,relative mechanical energy during short pulses (electrical excitationfrequency high compared to mechanical resonances) is minimized byincreasing η_(S), s, and m_(c) for a given B₀ and t_(g). Since m_(c)would be expected to increase as the second or third power of r_(g), itmight appear that motion-related problems are reduced by increasingr_(g). However, t_(g) (if inversely proportional to the gradient fieldstrength, B_(G)) will generally increase as the second, third, or evenfourth power of r_(g) in large coils, depending on the severity ofacoustic noise and the amount of power that can be justified. Thus,electro-mechanical efficiency in the short-pulse limit typicallyincreases with r_(g) because the pulse lengths must increase. For theGolay-crescent geometry of the instant invention, the constant in thedenominator is increased by a factor of four.

Another severe problem that often arises for the high-frequencycondition is that ω_(g), the dominant frequency component in thegradient pulses, may be close to ω_(b), the mechanical bowing modefrequency of the coilform, or close to one of the higher mechanicalmodes. The single pulse analysis is invalid near mechanical resonance asthe initial velocity is no longer zero. Thus, the actual situation isworse by a factor comparable to the mechanical Q, which has been foundto lie between 3 and 20 for typical materials and geometries.

The co-pending patent application Ser. No. 07/912,149 discloses themechanical advantages of small-diameter, solenoidal-like trapezoidalcoils external to the imaging region for reducing acoustic problems. Byincreasing the stiffness, it is then possible to stay below resonancefor the strongest couplings, and motion problems are easily eliminated.Also, the trapezoidal coil structures can achieve higher switchingefficiency but linearity is inferior unless novel winding densitiessimilar to those of the instant invention are used. The co-pendingpatent application also discloses that improved switching efficiency andimproved shielding can be obtained by interleaving Golay-type coils withtrapezoidal solenoidal coils.

According to equation 3!, coilform stiffness is not a factor aboveresonance. Mechanical energy is decreased by simply increasing the massof the coil. Hence, lead-filled copper tubing can be advantageous whenthe driving frequency is greater than a mechanical resonance. However,there will always be circumstances in which the coils are driven belowmechanical resonance. Thus, it is desirable to mount massive conductorson stiff coilforms so that mechanical efficiency is poor at both low andhigh frequencies.

A simple method of quantifying shielding effectiveness is to define fluxleakage Φ_(L) as the relative change in inductance when a passivecylindrical shield is added at radius r_(S), the radius of the radiationshields in the cryomagnet. This radius will typically be 1.2 (r_(g) +s).##EQU4## The above definition has only limited validity, as it does notdistinguish between the various field dependencies. The eddy currentsare far more sensitive to zero-order and first-order fields than to thehigh-order fields. The co-pending patent application Ser. No. 07/912,149discloses dynamic compensation of the low-order eddy currents.

FIG. 2 discloses the crescent coil of the instant invention, whichallows substantial improvements in switching efficiency, linearity, andshielding when used with Golay-type coils, especially when s is smallcompared to r_(g). Moreover, these improvements are achieved withoutsignificant increase in acoustic noise compared to the solenoidalgeometries. The thick-walled, crescent coilform 201 is made of ahigh-modulus, non-magnetic material of high electrical resistivity suchas glass-filled polyphthalamide (PPA) or a sialon. The crescent coilform201 comprises a cylindrical concave surface 202, a cylindrical convexsurface 203, radial surfaces 204, 205, and end surfaces 206, 207. Thetwo cylindrical surfaces are concentric and subtend similar azimuthalangles φ with respect to a common axis 208, but the convex surface 203will typically have greater axial length than that of the concavesurface 202 and may subtend a somewhat larger or smaller azimuthal angleφ. The radial surfaces and end surfaces 204, 205, 206, 207 aretrapezoidal in shape and may be inclined with respect to the radialdirection. The radial surfaces may be slightly convex to facilitate coilwinding. The axially oriented edges 211, 212, 213, 214 are radiused tosimplify coil winding, and grooves 215 and lips 216 may be added to thearcuate and radial surfaces for the same purpose.

Two parallel coil windings 220, 221 are shown as would be required insome cases, but single windings and multiple-layer windings would oftenbe preferred. For moderately large systems, lead-filled copper tubingmay be used for the conductor elements, where each conductor 220, 221 isfilled with lead 222. The increased density of Pb is beneficial inreducing acoustic noise and vibrations of the low-frequency transversemodes, according to equation 3!. In very large systems, the radial modesof the crescents could be more troublesome, in which case aluminum stripor tubing would be preferred for the windings. Always, the windingswould be securely bonded to the crescent coilform, preferably using afiber-reinforced thermosetting material, such as epoxy or polyester 223.The crescent is symmetric with respect to a reflection through a plane224 containing an internal coilform axis.

Novel fixturing is required to wind a coil on a coilform with a concavesurface. The technique illustrated in FIGS. 3a and 3b attaches acylindrical convex-convex spacer 301 to the concave surface 203 that canbe removed by etching, melting, dissolving, disassembly, or mechanicalmachining after the coil is wound. The convex winding elements 323 nearthe concave surface may then be bent inward and bonded to the concavesurface 202. The optimum winding density will always be higher on theconcave surface than on the convex surface 203, and it will often bedesirable to use two layers on the concave surface and a single layer onthe convex surface, which may require a separate winding fixturingoperation for each layer. For very large crescent coils, a moreconvenient approach than winding may be to solder individual segments ofproperly curved strips or tubing together. Then, different conductorelement sizes can be used on the different surfaces.

Another method of producing the required winding on a concave surfacewould be to start by coating the entire external surface of the crescentcoilform with copper film by sputtering, chemical vapor deposition,chemical precipitation, or any other suitable technique. A polymerresist can then be applied and developed by conventional screening orphotochemical processes to permit etching of the desired winding orwindings. The copper windings can then be electrochemically plated tothe desired thickness.

FIG. 4 depicts a thin-walled, nonmetallic, cylindrical coilform 401 ofapproximate outer radius r_(g) on which a hybrid y-gradient (δB_(Z) /δy)coil assembly of the instant invention is mounted. The hybrid coilassembly subtends azimuthal angle φ_(T) greater than 110° and less than150°, on each side of the x-z plane. Golay coil 410 has first azimuthalmembers 411 at mean polar angle θ₁ between 48° and 60° and secondazimuthal members 412 at mean polar angle θ₂ less than 42°. In FIG. 4,Golay coil 410 appears twice, once in plain view at the left of coilform 401, and again in phantom behind the coil form 401. The latter isshown with its upper winding portions (azimuthal members) 412 and withlower winding portions. The upper winding portions have a center whichdefines polar angle θ₂, shown in phantom when obscured by coil form 401and by part of winding portions 412. The lower winding portions, whichmirror region 411 in the figure, have a center which defines polar angleθ₁.

Six symmetrically y located crescent coils 420, 430, 440, and theirsymmetric counterparts on the -y side, are used in place of thesolenoidal coils of the co-pending patent application Ser. No.07/912,149.

The crescents have concave conductor elements 421 at approximate radiusr_(g), convex conductor elements 422 at approximate radius r_(g) +s, andradial conductor elements 423 therebetween.

Diagonal crescents 420, 440 are positioned between the x and y axes withcenterline φ at 45° and 135° respectively and subtend azimuthal anglesof φ_(d) each. Center crescent 430 is centered with respect to the yaxis and subtends azimuthal angle φ_(c), where φ_(d) +φ_(c) ≈86°.Typically, φ_(d) and φ_(c) are each approximately 43°.

The axial length h₁ on the concave side of the crescents (See FIG. 2) isgreater than r_(g) /2 and less than 1.5r_(g). The axial length h₂ on theconvex side of the crescents is greater than 1.2h₁ but less than 2.5h₁.Numerical integration of the Biot-Savart law may be used to optimizewindings for various values of s/r_(g) and various optimizationcriteria. The surface current density on the concave side of the centercrescent is typically about 40% greater than that on the diagonalcrescents. Surface current densities on the convex sides of thecrescents are typically lower near their ends than near the center.Surface current densities on the concave sides of the crescents areoften 20% higher near their ends than near the center. Surface currentdensities in the Golay coils are typically about 20% higher than thehighest current densities in the crescents.

Since the crescent conductor elements form complete loops and aresymmetrically inclined with respect to the uniform external field B₀,there are no net Lorentz forces or torques on the center-of-mass of anyof the crescents to first order. Thus, it is not necessary to beconcerned about torsional or positional rigidity of the crescents in thegradient assembly. It is sufficient that the crescent coilforms be madeof a material of high modulus to insure that the radial-mode mechanicalresonances in the crescents are not excited. The crescents may then beprecisely secured in position by epoxy bonding the concave surfaces ofthe crescents to the surface of the cylindrical coilform 401 withoutundue concern about rigidity of this cylinder. The elastic modulus ofcoilform 401 can be as low as 3 GPa in some cases, but elastic modulusabove 200 GPa would be preferred for large systems in high B₀ tominimize motion-related problems from the Golay coils at lowfrequencies.

Forced air cooling over the surfaces of the crescents and Golay coilswill often provide sufficient cooling, but in some applicationsadditional cooling will be required. FIG. 5 depicts an effective methodof achieving higher power density by providing the benefits of watercooling with fewer of the electrical isolation and plumbing problemsassociated with conventional water cooling methods. (One prior arttechnique is to bond plastic plumbing to the surface of the fingerprintcoils. Another prior-art technique is to hermetically coat allconductors and then flood the coil assembly with water. Another priorart technique is to use copper tubing for the coils and circulatedeionized water directly through the live windings). The thin-walledcoilform 401 is made from a material having high thermal conductivityand high strength, preferably silicon nitride, or alumina, or analumina-filled composite. An inner thin-walled cylinder 501 of innerradius r₁ and outer radius r₂ is used to establish an internal coolingwater jacket. O-rings 502, 503 may be used for compliant sealing, andplastic plumbing 504 can be used to circulate water through the annularspace between the concentric cylinders. The Golay coil 410 bonded to theouter surface of the coilform is easily cooled by conduction. Thecrescents 420, 430, 440 are well cooled on their concave surfaces, andthe high thermal conductivity of the heavy copper windings will usuallyconduct sufficient heat for adequate cooling of the other surfaces ofthe crescents. Both forced air cooling and water cooling may be used,allowing more flexibility in the use of the coils. The use ofhigh-modulus, high-strength materials, allows the total relativethickness, (r_(g) -r₁)/r_(g), to be less than 0.1.

It will normally be desirable to mount the X, Y, and Z gradients on asingle coilform of approximate radius r_(g) as shown in a perspectiveview in FIG. 6a and in a cross section of the plus-plus quadrant of theyz plane in FIG. 6b. Note that x-Golay coil 610 and y-Golay coil 615overlap and that diagonal crescents 620, 630, 640 are used for both thex and y axes. The x-Golay coil 610 and the y-Golay coil 615 areessentially identical except that one is positioned at φ+90° relative tothe other. Clearly, one is also at a slightly larger radius; the zposition may also be shifted slightly. Conventional, corrected Maxwellpairs 660 are wound over the Golay coils for the z gradient. Theinternal water jacket 505 effectively cools all of the windings. Somesimplification in the winding of the outer Golay coil (and perhaps someimprovement in the cooling of the outer Golay coil) may be obtained byadding spacer material 611, equal in thickness to that of the innerGolay coil, to the surface of the coilform 401 beyond the azimuthalextent of the inner Golay so that the outer Golay has a substantiallysmooth, cylindrical surface on which to be mounted. For best heattransfer without eddy current problems, this spacer material wouldconsist of a large number of open-ended segments of copper wires.

Optimum positions of external shielding coils 661 for the quadrupolarz-gradient may be determined by the method of Beth, U.S. Pat. No.3,466,499, or by numerical application of the Biot-Savart law, or byother well-known methods. The octopolar fields produced by theGolay-crescent transverse gradient coils have extremely low externalfields, but further shielding Golay coils 616 may still be desired.(Note that these coils are not shown in the isometric view). Numericalapplication of the Biot-Savart law is most practical for calculatinggradient fields, leakage flux, and total inductance (from the totalmagnetic energy).

There are two general methods of orthogonally powering the diagonalcrescents. In the first method, the center crescents each have onewinding and the diagonal crescents each contain two sets of parallelwindings, one set for each transverse axis. Each center crescent wouldtypically have current density 40% greater than that of a single one ofthe windings on a diagonal crescent. This method has the advantage thateach axis requires only one power amplifier. The x-amplifier drives thetwo crescents on the x-axis and one of the winding sets on each of thefour diagonal crescents. The y-amplifier drives the two crescents on they-axis and the other winding set on each of the diagonal crescents. Onepossible schematic representation of this approach is shown in FIG. 7.Here, each winding set consists of a single, full-length winding; the +zends of the crescents are shown with dots, and all of the windings arecounterclockwise when viewed from the +z end. In this case, the centercrescents L_(C) are in series with paralleled diagonal crescents L_(D),but many other series-parallel arrangements can be devised that producethe desired current densities and maintain orthogonality--that is,result in zero mutual inductance between the axes. The Golay coils aremost conveniently wired in series with the crescents on each axis, as itis difficult to accurately calculate the separate inductances, but theycan be paralleled with the crescents if the individual and mutualinductances can be accurately determined and matched along with properresistance match. Of course, the fourfold symmetry allows simpleparallel arrangements of the four quadrants with all coils per quadrantin series.

In the second general method, each diagonal crescent has only onewinding set that is shared by both axes, but four power amplifiers arerequired to drive the transverse axes. For purposes of illustration, thefour amplifiers are assumed to have equal transconductance and thecrescents have equal turns densities. This approach has the advantagesof simplified crescent windings and interconnections, easier impedancematching, reduced interwinding capacitance, and higher efficiency. Onepossible schematic representation to this approach is shown in FIG. 8using the same conventions as used in FIG. 7. Again, manyseries-parallel variations are possible that achieve the desired surfacecurrent densities on the crescents. The X and Y signals are summedbefore being fed into the X+Y amplifier, and their difference is fedinto the X-Y amplifier. While such a configuration requires a minimum offive amplifiers for an X-Y-Z gradient system compared to three forconventional designs, this approach would often be less expensive inlarge systems, where amplifier cost is primarily determined by totalpower. The amplifiers for the center crescents would require higherpower rating and perhaps higher output impedance than the diagonalgradient amplifiers since the Golay coils would be connected in serieswith the center crescents. The voltages driving the diagonal crescentsand the diagonal turns densities may, of course, be multiplied by anyconvenient factor to allow the use of amplifiers of more convenientimpedances at the different power levels while achieving the desiredrelative current densities. In large systems, eight amplifiers wouldusually be used in a balanced configuration.

The embodiment of FIGS. 4 through 8 uses eight crescents for an X-Ygradient system. Some additional improvement in both switchingefficiency and linearity may be obtained in large systems throughfurther azimuthal segmentation for further adjustment of the azimuthalcurrent densities--but with increased complexity and higher cost.

FIG. 9 is a cross section through the z=0 plane of an X-Y gradient coilsystem using 12 crescents. In general, 4n crescents may be used, where nis an integer and the azimuthal current densities in the distributedcrescents approximate that of a sine function. For 12 crescents centeredat azimuthal positions φ=0°, 30°, 60°, 90°, 120°, 150°, . . . , they-current density at 60° and 120° is approximately 87% of that at 90°and the y-current density at 30° and 150° is approximately half that at90°. For the case of twelve or more crescents azimuthally, it becomesimpractical to achieve the desired current densities on both axesthrough multiple windings driven by one amplifier per axis in a similarmanner to that shown in FIG. 7. However, the method of FIG. 8 using asingle winding per crescent can easily be extended to any number ofcrescents.

FIG. 10 is a schematic representation of an arrangement using sixamplifiers to drive twelve crescents for an X-Y system. The centercrescents are wired in series with the Golay coils, and mixtures of thex and y signals are supplied to the off-axis crescents. Again, therelative voltages (or currents) and the turns densities may be scaledappropriately for more convenient amplifier impedances. The impedancesof each crescent in the set at azimuthal positions φ, φ+180°, -φ, and180°-φ would be identical, but not necessarily related to the impedancesof any other set of symmetrically related crescents by any relationshipother than amplifier economics. Since the off-axis crescents wouldrequire lower power than the center crescents (especially when theGolays are in series with the center crescents), the off-axis crescentswould normally be of higher impedance. That is, the off-axis crescentswould normally have higher turns density, even though they would beoperated at lower current density.

The crescent concept as a means of confining magnetic flux may beextended to more complex geometries with some additional improvement inboth switching efficiency and shielding effectiveness but with increasedcomplexity in the Biot-Savart numerical optimization, the manufacturingof the coilform, and in the coil winding or etching process. FIG. 11shows one such extension of the crescent concept. The external concavecylindrical surface is symmetrically stepped to form a mid-externalconcave cylindrical surface 1120 with radius r_(g) and two symmetricallylocated end-external concave cylindrical surfaces 1121, 1122 at radiusr_(g) +ε, where ε is small compared to r_(g), so that the azimuthalwindings on surfaces 1121, 1122 may overlap other coils on systemcoilform 401 at these axial locations. For example, it may be desirableto have a Z gradient winding 140 in this location to improve thelinearity of the Z gradient. In such case of a Z gradient 1141 woulddesirably fit in stepped area 1126 and 1127. The external convexcylindrical surface may also be symmetrically stepped to form amid-external convex cylindrical surface 1125 at radius r_(g) +s and twosymmetrically located end-external convex cylindrical surfaces 1126,1127 at radius r_(g) +s-ε. The crescent ends include annular sectors1131, 1132, on which are mounted end-azimuthal conductors. Portions ofhyperbolic surfaces of revolution 1133, 1134 join the end annularsectors to the end-concave cylindrical surfaces. Among the surfaces withazimuthal current conductors, the mean surface current density would behighest on the end-external concave cylindrical surfaces and lowest onthe annular sectors. The windings of the crescent coil are desirablyangled as shown in FIG. 11(b) so as to enhance flux confinement, thus,minimizing axial flux leakage and eddy currents in the magnet.

Although this invention has been described herein with reference tospecific embodiments, it will be recognized that changes andmodifications may be made without departing from the spirit of thepresent invention. All such modifications and changes are intended to beincluded within the scope of the following claims.

We claim:
 1. An MRI gradient coil system substantially symmetric withrespect to a 180° rotation about a central axis, substantially symmetricwith respect to a first plane perpendicular to said central axis, andsubstantially symmetric with respect to a reflection through a secondplane containing said central axis, said system comprising:anonmetallic, nonmagnetic innermost gradient coilform cylinder of outerradius r_(g) and inner radius r₃ ; electrically conductive elementsbonded to the outer surface of said coilform cylinder; a concentricinner cylinder of inner radius r₁ and outer radius r₂, where r₂ is lessthan r₃ ; water sealing means between said cylinders at both endsthereof; and plumbing means to permit fluid flow in the annular spacebetween said cylinders.
 2. The coil system of claim 1 in which saidcoilform cylinder is made from a material having thermal conductivitygreater than 3 W/mK, elastic modulus greater than 12 GPa, and rupturestrength greater than 100 MPa; said system further characterized suchthat (r_(g) -r₁ /r_(g) is less than 0.1.
 3. The system of claim 1wherein the cylindrical coilform comprises silicon nitride.
 4. Thesystem of claim 1 wherein the cylindrical coilform comprises alumina. 5.The system of claim 1 wherein the cylindrical coilform comprises analumina-filled composite.
 6. An MRI gradient coil system substantiallysymmetric with respect to a 180° rotation about a central axis,substantially symmetric with respect to a first plane perpendicular tosaid central axis, and substantially symmetric with respect to areflection through a second plane containing said central axis, saidsystem comprising:a nonmetallic, nonmagnetic coilform cylinder of outerradius r_(g) and inner radius r₃ ; electrically conductive elementsbonded to the outer surface of said coilform cylinder; a concentricinner cylinder of inner radius r₁ and outer radius r₂, where r₂ is lessthan r₃ ; water sealing means between said cylinders at both endsthereof; and plumbing means to permit fluid flow in the annular spacebetween said cylinders,in which said coilform cylinder is made from amaterial having thermal conductivity greater than 3 W/mK, elasticmodulus greater than 12 GPa, and rupture strength greater than 100 MPa;said system further characterized such that (r_(g) -r₁)/r_(g) is lessthan 0.1.
 7. The system of claim 6 wherein the cylindrical coilformcomprises silicon nitride.
 8. The system of claim 6 wherein thecylindrical coilform comprises alumina.
 9. The system of claim 6 whereinthe cylindrical coilform comprises an alumina-filled composite.
 10. AnMRI gradient coil system substantially symmetric with respect to a 180°rotation about a central axis, substantially symmetric with respect to afirst plane perpendicular to said central axis, and substantiallysymmetric with respect to a reflection through a second plane containingsaid central axis, said system comprising:a nonmetallic, nonmagneticinnermost gradient coilform cylinder of outer radius r_(g), inner radiusr₃, thermal conductivity greater than 3 W/Km, and elastic modulusgreater than 13 GPa; electrically conductive elements bonded to theouter surface of said coilform cylinder; a concentric inner cylinder ofinner radius r₁ and outer radius r₂, where r₂ is less than r₃ ; watersealing means between said cylinders at both ends thereof; and plumbingmeans to permit fluid flow in the annular space between said cylinders.11. The system of claim 10 wherein the cylindrical coilform comprisessilicon nitride.
 12. The system of claim 10 wherein the cylindricalcoilform comprises alumina.
 13. The system of claim 10 wherein thecylindrical coilform comprises an alumina-filled composite.